1. Field of the Invention
The present invention relates to a structure of a lens apparatus of camera which takes an object image.
2. Description of the Related Art
The number of pixels of an image pickup element (CCD, etc.) for an electronic camera represented by a digital camera is significantly increasing in recent years and its pixel pitch has a tendency to reduce rapidly. On the other hand, there is a lens barrel of a camera having an iris diaphragm having a stop blade with an opening or a plurality of stop blades, gradually changing the stop diameter to restrict the quantity of light incident on an image plane and arranged in an optical path.
Light has properties of a wave and it is a well-known fact that these properties become conspicuous when the diameter of the aperture stop falls below a certain value, that is, a phenomenon of diffraction takes place. Here, when the pixel pitch of the image pickup element is reduced, the image pickup element can even capture a high frequency area, but in this high frequency area it receives strong influences of diffraction and resolution (image quality) deteriorates drastically.
For this reason, for the purpose of suppressing deterioration of the image quality, it is not possible to reduce the aperture of the stop when the pixel pitch of the image pickup element is small.
Therefore, when the aperture is small, a structure in which an ND (Neutral Density) filter structured integral with the stop blade is moved into the optical path is used. The ND filter has an effect of restricting the amount of transmitted light and can thereby perform effective light quantity control without the need to reduce the aperture of the stop and suppress aforementioned deterioration of the image quality.
Restricting the light quantity is not simply limited to the effect of appropriate exposure, but also has the effect of actually adding various effects to photographic expressions by a photographer, which constitutes an important factor. For example, photographic expressions using various image taking methods are required such as highlighting an object by fully opening the stop, clearly describing all parts of scenery with the stop stopped down, increasing the shutter speed with an increased light quantity, slowing down the shutter speed with a reduced light quantity or the like.
However, the stop provided with an ND filter as an integral part is subject to many restrictions on photographic expressions such that there is only one diameter of a reduced aperture to adjust the light quantity with the ND filter or it is not possible to reduce the light quantity with the stop fully opened. That is, the conventional ND filter is used to suppress deterioration of the image quality in a small aperture state and not intended to actively create or edit pictures.
Thus, there is a demand for a lens barrel having an iris diaphragm capable of a multi-stage setting of the stop diameter and a ND filter adjusting the quantity of transmitted light respectively, but it is difficult to arrange a driving unit for the stop and a driving unit for the ND filter within the limited space in the lens barrel.
Furthermore, when the stop unit, shutter unit and ND unit are held in the same lens holding frame for a purpose of reducing a size of the lens barrel, the overall weight of the lens holding frame increases, which causes a problem that when external shock is given to the lens barrel, the lens holding frame is tilted with respect to the optical axis because of this weight.